CAMs are memories in which data is selected based on its contents, rather than its physical location. This function is useful for many applications, especially when performing a look-up for the purposes of mapping from a long identification word to a shorter word. This operation is required in many telecom functions, including Asynchronous Transfer Mode (ATM) address translation.
U.S. Pat. No. 5,289,403 entitled "Self-Timed Content Addressable Memory Access Mechanism with Built-In Margin Test Feature" granted to J. D. Yetter on Feb. 22, 1994 describes a means of providing self-timing to a NOR-match line CAM, using a dummy column and dummy row. The bit at the intersection of the dummy column and dummy row always misses, while all other cells in that row always match, and this generates the slowest possible mismatch condition on the dummy match line. This signal is then used to generate a clock for timing of subsequent events, and for qualifying all other match lines.
U.S. Pat. No. 5,453,948 entitled "Associative Memory" granted to M. Yoneda on Sep. 26, 1995 describes a low-power implementation of a match line, in which the match line is realized as a series chain, instead of a NOR.
U.S. Pat. No. 5,455,784 entitled "Associative Memory Device with Small Memory Cells Selectively Storing Data Bits and Don't Care Bits" granted to H. Yamada on Oct. 3, 1995 describes a CAM with individual storage cells comprised of a series combination of two EEPROM devices. The series connection of these cells (one per bit) to form a word of CAM results in a series match line chain. Shifted transistor threshold voltages, resulting from programming, cause the EEPROM devices to act as either enhancement- or depletion-mode transistors, and hence "don't cares" may be stored by making both devices depletion-mode.
U.S. Pat. No. 5,469,378 entitled "Content Addressable Memory Having Match Line Transistors Connected in Series and Coupled to Current Sensing Circuit" granted to R. Albon et al on Nov. 21, 1995 describes a series match line chain.
Most previous CAMs employ a dynamic wired-NOR match line pull-down, which provides high speed. However, the prior art circuit construction causes match lines associated with mismatched words to experience transitions, while no transitions occur on match lines associated with matched words. Since the number of matches is much smaller than the number of mismatches, the number of transitions, and hence the power dissipation, is excessive for a wired-NOR match line CAM.